The colon is inhabited by a dense population of microorganisms, the so-called “gut microbiota”, able to ferment carbohydrates and proteins that escape absorption in the small intestine during digestion. This microbiota produces a wide range of metabolites, including short chain fatty acids (SCFA). These compounds are absorbed in the large bowel and are defined as 1-6 carbon volatile fatty acids which can present straight or branched-chain conformation. Their production is influenced by the pattern of food intake and diet-mediated changes in the gut microbiota. SCFA have distinct physiological effects: they contribute to shaping the gut environment, influence the physiology of the colon, they can be used as energy sources by host cells and the intestinal microbiota and they also participate in different host-signalling mechanisms. We summarize the current knowledge about the production of SCFA, including bacterial cross-feedings interactions, and the biological properties of these metabolites with impact on the human health

Probiotics are live microorganisms which when administered in adequate amounts confer a health benefit on the host. The main probiotic bacteria are strains belonging to the genera Lactobacillus and Bifidobacterium, although other representatives, such as Bacillus or Escherichia coli strains, have also been used. Lactobacillus and Bifidobacterium are two common inhabitants of the human intestinal microbiota. Also, some species are used in food fermentation processes as starters, or as adjunct cultures in the food industry. With some exceptions, antibiotic resistance in these beneficial microbes does not constitute a safety concern in itself, when mutations or intrinsic resistance mechanisms are responsible for the resistance phenotype. In fact, some probiotic strains with intrinsic antibiotic resistance could be useful for restoring the gut microbiota after antibiotic treatment. However, specific antibiotic resistance determinants carried on mobile genetic elements, such as tetracycline resistance genes, are often detected in the typical probiotic genera, and constitute a reservoir of resistance for potential food or gut pathogens, thus representing a serious safety issue.

The pool of microbes inhabiting our body is known as “microbiota” and their collective genomes as “microbiome”. The colon is the most densely populated organ in the human body, although other parts, such as the skin, vaginal mucosa, or respiratory tract, also harbour specific microbiota. This microbial community regulates some important metabolic and physiological functions of the host, and drives the maturation of the immune system in early life, contributing to its homeostasis during life. Alterations of the intestinal microbiota can occur by changes in composition (dysbiosis), function, or microbiota-host interactions and they can be directly correlated with several diseases. The only disease in which a clear causal role of a dysbiotic microbiota has been demonstrated is the case of Clostridium difficile infections. Nonetheless, alterations in composition and function of the microbiota have been associated with several gastrointestinal diseases (inflammatory bowel disease, colorectal cancer, or irritable bowel syndrome), as well as extra-intestinal pathologies, such as those affecting the liver, or the respiratory tract (e.g., allergy, bronchial asthma, and cystic fibrosis), among others. Species of Bifidobacterium genus are the normal inhabitants of a healthy human gut and alterations in number and composition of their populations is one of the most frequent features present in these diseases. The use of probiotics, including bifidobacteria strains, in preventive medicine to maintain a healthy intestinal function is well documented. Probiotics are also proposed as therapeutic agents for gastrointestinal disorders and other pathologies. The World Gastroenterology Organization recently published potential clinical applications for several probiotic formulations, in which species of lactobacilli are predominant. This review is focused on probiotic preparations containing Bifidobacterium strains, alone or in combination with other bacteria, which have been tested in human clinical studies. In spite of extensive literature on and research into this topic, the degree of scientific evidence of the effectiveness of probiotics is still insufficient in most cases. More effort need to be made to design and conduct accurate human studies demonstrating the efficacy of probiotics in the prevention, alleviation, or treatment of different pathologies.

Microbial colonization of the infant gut is essential for the development of the intestine and the immune system. The profile of intestinal microbiot

To assess the establishment of the intestinal microbiota in very low birthweight preterm infants and to evaluate the impact of perinatal factors, such as delivery mode and perinatal antibiotics. We used 16S ribosomal RNA gene sequence-based microbiota analysis and quantitative polymerase chain reaction to evaluate the establishment of the intestinal microbiota. We also evaluated factors affecting the microbiota, during the first 3 months of life in preterm infants (n = 27) compared with full-term babies (n = 13). Immaturity affects the microbiota as indicated by a reduced percentage of the family Bacteroidaceae during the first months of life and by a higher initial percentage of Lactobacillaceae in preterm infants compared with full term infants. Perinatal antibiotics, including intrapartum antimicrobial prophylaxis, affects the gut microbiota, as indicated by increased Enterobacteriaceae family organisms in the infants. Prematurity and perinatal antibiotic administration strongly affect the initial establishment of microbiota with potential consequences for later health.

Cross-feeding is an important metabolic interaction mechanism of bacterial groups inhabiting the human colon and includes features such as the utilization of acetate by butyrate-producing bacteria as may occur between Bifidobacterium and Faecalibacterium genera. In this study, we assessed the utilization of different carbon sources (glucose, starch, inulin and fructooligosaccharides) by strains of both genera and selected the best suited combinations for evidencing this cross-feeding phenomenon. Co-cultures of Bifidobacterium adolescentis L2–32 with Faecalibacterium prausnitzii S3/L3 with fructooligosaccharides as carbon source, as well as with F. prausnitzii A2–165 in starch, were carried out and the production of short-chain fatty acids was determined. In both co-cultures, acetate levels decreased between 8 and 24 h of incubation and were lower than in the corresponding B. adolescentis monocultures. In contrast, butyrate concentrations were higher in co-cultures as compared to the respective F. prausnitzii monocultures, indicating enhanced formation of butyrate by F. prausnitzii in the presence of the bifidobacteria. Variations in the levels of acetate and butyrate were more pronounced in the co-culture with fructooligosaccharides than with starch. Our results provide a clear demonstration of cross-feeding between B. adolescentis and F. prausnitzii.